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Cars and Trucks

Where the Rubber Meets the Road … or, How Come Trojan Doesn’t Make Tires?


If they did, I’d buy them. Whether it’s tires for your car or … well, you know … freshness counts. Prophylactics have a “use by” date. Tires should too. Fortunately, there is a way to know if the rubber that you’re trusting with your life is going to hold together and not fail at an inopportune moment. Hey! Snap out of it! What are you, a Delta house pledge? I’m referring to your car’s tires. 

When I buy tires for a car I intend to keep, I don’t want the cheapest tires.  There’s a reason they’re so cheap and it usually doesn’t match up with the concept of long-term satisfaction.  Instead, I try to get the best tires for the job at the lowest price, always checking to be sure they’re not past their freshness date.  So, to paraphrase Steve Martin in Father of the Bride, fasten your condoms … uh, seat belts.  Here are the three things you need to know about buying tires.

1.  The DOT (Department of Transportation) stamping on every tire sort of tells you what you need to know about freshness.  Among other things, it tells you when the tire was made. Unfortunately there is no definite guideline for how old a tire can be before it is no longer roadworthy. But consider this, would you put more trust in a tire that was manufactured this year or one that sat on a shelf for six or eight years? Hint: tire rubber has little in common with fine wine. Tires are best when fresh and spend the rest of their lives trying to biodegrade – in spite of what environmentalists tell you.

The DOT code is printed on the sidewall of every tire. Sometimes it’s only on the inner sidewall. It’s not supposed to be that way, but sometimes it is.  In other words, you can’t always see it once the tire is mounted on a vehicle without hoisting or crawling underneath. So, check it out before you purchase the tires. And be sure the ones you approve are the ones that actually get mounted on your vehicle. The first several digits are to identify what factory manufactured the tire and for batch tracking in case of recall.  The last four digits at the end of the DOT code spell out the date. The first two of the four are the week of the year and the second two are the last two digits of the year the tire was made. For example, a DOT code ending in 0609 would mean the tire was manufactured the sixth week of 2009 … sometime in February. If you ever find one with only three digits, keep your distance and don’t get any of that soon-to-be-disintegrating rubber on you. It was made in the 1990’s. It’s now old enough to be dangerous. And in case you’re an incurable bargain shopper, remember – lower price does not make up for an aged tire that’s far more likely to fail with very ugly consequences.

2.  You also want to be sure you’re getting the right size tire for your vehicle.  Best rule of thumb is to stick with the size that’s currently on the vehicle, especially if it still has its original tires.  These are tires that you can be sure will not come in contact with fenders or foul up your speedometer calibration.  They were also chosen by the manufacturer to best match the character of your vehicle.  Note that it’s not necessary or perhaps even desirable to keep the same brand or model if you want to change that character a little.  The tire you should choose depends on whether your needs run towards economy, performance, ride or foul weather safety. 

Tire size is expressed as a series of numbers and letters.  Take the example of P225/60 R18 93S.  The P means this tire is P-metric and is designed for passenger car use.  Other letters you might see instead of P  include LT for light truck and T for temporary spare.  The 225 is the width of the tire in millimeters at its widest point.  On most tires, this is roughly equivalent to tread width.  60 is the aspect ratio of sidewall to tread.  In this case, the sidewall (wheel rim to tread surface distance)  is 60 percent of 225 millimeters.  R indicates a tire carcass that has the now almost universal radial cord structure.  And 18 is the diameter of the wheel this tire fits. 

 You might also see the M+S designation for mud and snow suitability in front of the entire string of letters and numbers if the tire is so designed.   The digits conclude with the “Service Description” which includes two or three numbers that identify the load rating and a letter indicating the speed rating.  The load rating can range from 71, equating to 761 pounds, up to 125, equating to 3690 pounds.  The speed ratings range from Q, meaning the tire is good for use up to 100 miles per hour, progressing up to W, for up to 168 mph.  There is also a Z rating that stands for “over” 149 mph.  The “93S” in our example above means this tire can handle 1433 pounds at up to 112 mph when properly inflated.  Again, the service description appears after the wheel diameter. 

Note that the string of numbers and letters like our example of P225/60 R18 93S may appear without the service rating as “advertising” along with the manufacturer’s name and tire model name.  But you should be able to find the entire sequence, probably in smaller print, somewhere on the sidewall.  Quite a nice litlle alphabet soup, eh?  But wait, there’s more!

When swapping out wheels to obtain higher performance, there is a formula your tire store will apply to find the correct overall diameter involving increasing the tread width and decreasing the aspect ratio to match wheels that are wider and of a greater diameter.  It’s called Plus Sizing.  If you’re not changing wheel diameter, the only thing you need to know regarding changing  tire sizes is:  To maintain the same total outside diameter of wheel/tire assembly, you need to decrease the aspect ratio by 5 percent for every 10 millimeters increase in width.  In other words, a P215/60 R16 tire has the same overall diameter (will turn the same number of revolutions per mile) as one that is size P195/70 R16.  In this example, twenty millimeters wider requires ten points lower aspect ratio to get a tire that’s the same overall diameter.  That  is generally about as much wider a tire as you’re going to want go without getting wider wheels.  Got all that? 

3.  Finally, the UTQG, or “uniform tire quality gradings” are on the side of the tire and give you an idea of how that tire compares to a theoretical base tire.  These are the three T’s of tire life – treadwear, traction and temperature.  The number is the treadwear or relative mileage life of a tire compared to any other tire – higher number means it’ll last more miles.  It also probably means less sticky and therefore, not the best choice for high performance cornering or braking.  The next letter or letters signify traction.  A or AA is best, B not as good and C, I think means Conestoga wagon wheel.  Traction is accomplished by a combination of stickiness and tread design.  Summer, or non-mud and snow tires have less space between the blocks of tread rubber.  They have good dry weather traction, sometimes at the expense of tread life or foul weather safety.  But in good weather, they stick like glue compared to a winter/foul weather or off-road tire.   The final letter is temperature, or how hot the tire gets.  As with batteries, heat is a passenger car tire’s enemy.   Note that I say passenger car.  Because race cars that run on “gumballs” like to run hot.  Heat makes them stickier – up to a point.  Generally, a tire that runs hot will break down sooner – sometimes with catastrophic results.  Look for a tire with a temperature rating of A … or at minimum, B. 

Now, how do you put all this knowledge to use when selecting a tire for your car?  Every situation is different.  But here’s an example with the reasoning for the selection I made.  A few years back I re-shod a Chevy Metro with Michelin X’s – probably about the most expensive tire available for this economical demi-car … retail price almost $100 per tire. I upgraded from the car’s stock 155/80R13’s to 175/70R13, which the car’s owners manual confirmed was the correct optional size. (20 millimeters wider, 10% lower aspect ratio, right?)  I bought them at Costco and during a sale – $20 off per tire – which is a big discount on a small-size tire. The tire had UTQG ratings of 580AB, had a M+S rating and came with an 80k mile tread-life guarantee. High performance in any terms other than fuel economy is a stranger to this little econo-car, so I wanted tires that would give the car a little more stability, and would last forever.  That’s why I chose a wider size and was mainly concerned with the treadwear number.   Service description (weight capacity and maximum speed) ratings were not a factor because even the lightest rated tires would handle twice the weight of this car and the little 1.0 liter three-cylinder engine is maxed at 85 mph … flat-out … downhill … with a tailwind… bless its little heart.  Because Costco is a volume store, the tires were fresh … less than six months old. Again, this is important. I’ve seen tires in stores with DOT codes showing they were over six years old. That’s not acceptable for a tire. That’s an age at which you ought to consider replacing a tire, not buying it.  My total cost for those four Michelins with mounting and balance was about $200.

Now, I could have gotten cheaper stock-size tires for as little as $120 at another chain. But based on their lower UTQG statistics and narrower size, they would have worn out in half the time and not performed as well.  In other words, they weren’t worth the lower initial price … not if I plan on keeping the car.  And the upgrade in quality and contact area – where the rubber meets the road (see how everything ties in?) – absolutely transformed that little Metro into something much more stable and comfortable to drive. That was the best money I ever spent on a car.  The best money I ever spent for use in a car was for a box of Trojans and a bottle of Boone’s Farm Strawberry Hill.  But that’s another story.

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